There is a recognized need for the detection of undesirable concentrations of potentially harmful airborne bacteria in health care environments, laboratories and in warfare conditions. Processes for the detection of airborne particles, which may contain harmful bacteria such as anthrax (bacillus anthracis), typically comprise sizing and characterizing the particles as biological. As bacteria can clump together, the term "particle", used hereinafter, is understood to include inert particles, a single biological entity or biological (typically 1 .mu.m), or an aggregate of these small biologicals (aggregates of about 3-10 .mu.m).
Means for measuring a particle's size include analysis of light scattering, electrical mobility, or particle inertia in an accelerated fluid flow. First, a particle's size is indicative of its potential as a respiratory hazard. Secondly, whether it is biologically hazardous or not requires further determination of the particles composition. Composition or biological characteristics are typically determined using reagent-bases processes (such as the detection of biological iron) or apparatus such as a flow cytometer.
The sizing of particles is readily performed in real-time, but the determination of a particle's composition is generally performed using time consuming, off-line analysis. In events where one or more particles may pose extraordinary risks to humans, such as bacteria and their spores, off-line analysis may provide results too late to make an appropriate and safe response.
The sizing of particles using aerodynamic principles is a conventional technique used widely to obtain high resolution size distributions of particles in the range of 0.5 to 15 .mu.m. As described in the paper Performance of a TSI Aerodynamic Particle Sizer, Aerosol Science and Technology, 1985, 4:89-97 by Yeh et al., an air sample containing particles is drawn through a small nozzle that produces an abrupt acceleration in the airstream. The resulting velocity of a particle as it exits the nozzle is dependent upon its inertia and its acceleration in the fast airstream. Therefore, the end velocity of very small particles corresponds nearly with the velocity increase of the air stream, while larger, higher inertia particles resist the acceleration and have a lower end velocity.
Particle end velocities are measured very near the nozzle exit by measuring the time between the particle's sequential interruption of two, closely spaced laser beams just beyond the nozzle's exit. This technique is capable of usefully determining particle sizes in the respirable range of about 0.5 .mu.m to 15 .mu.m.
Applicant has previously employed commercial aerodynamic particle sizing (APS) instruments for rapid detection of "possible" bioaerosol events as disclosed in applicant's 1994 paper Detection of Biological Warfare Agents, Today Science Tomorrow Defence, pp.11-18, Ed. C. Boulet, Government of Canada Cat. No. D4-1/175E. As described, these possible events have heretofore only been inferred from the concentrations of particles, in particular size ranges, as being generally consistent with the presence of biologicals. The prevailing disadvantage for applying an APS apparatus for bioaerosol detection lays in its inability to unequivocally distinguish whether the particle is biological or not. Characteristics other than size are needed to distinguish harmful biological particles from inert (ie. mineral) particles.
Conventional liquid flow cytometers have grown in use over the last ten years to become an important tool for the characterization of micro-biological specimens. Flow cytometry is well understood in the art, as described in the paper, How a flow Cytometer Works, Practical Flow Cytometry, 1988, p. 84, A. R. Liss, Inc. NY, N.Y. by Shapiro et al. Basically, the flow cytometer projects a laser light at a moving stream of particles conveyed in a liquid carrier and uses the resultant light scatter or fluorescence to identify individual particle's characteristics.
In optical instruments, the presence of air interposed between the viewing optics and the particle degrades the viewing quality. Thus, in the flow cytometer, the particles are first prepared by immersion in an optically transparent fluid. Further, in some cases, particles are labelled with a fluorescent tag, such as by tagged monoclonal antibody-antigen reactions. The particle-laden liquid is passed through one or more overlapped laser-light beams. Light which scatters upon contact with the particles is measured and any fluorescence is detected in one or more emission wave bands.
By measuring the forward scatter and applying pre-determined criteria, the particle size may be determined. Using the side scatter, a particle's cellular constituents may be determined. Lastly, fluorescence of the particle indicates presence of the expected fluorescent antibody, and more particularly, the antigen it is bound to.
However, fluorescent tagging is not practical for aerosol sampling as one cannot tag all the incoming airborne particles on a real-time basis. Therefore, one must instead investigate the intrinsic characteristics of particles themselves. Further, a flow cytometer requires both pre-preparation of the particulate matter in an optically-enhancing liquid carrier, and foreknowledge of the characteristics of the particles of interest (so as to attach an appropriate fluorescent tag). Simply, the flow cytometer does not accept an atmospheric air stream sample for analysis in real-time.
In summary, the APS apparatus is not able to discern between hazardous biological and inert, respirable particles and a flow cytometer is unable to process airborne particles.
In an alternate, reagent-based biological characterization process, continuous use of consumable reagents can become problematical and prohibitively expensive.
Therefore, the present invention is directed towards correcting the deficiencies of the known apparatus so as to enable an operator to perform real time detection of bioaerosol hazards in the respirable size range, and to establish whether they are inert or biological.